3
This depends upon a number of things including your server settings, php settings and XOOPS settings.
In general, there are two ways to deal with sessions. First is in PHP (I forget what version this started in). There is a set of variables that deals with sessions and you can make changes to that if available.
However, the easier way to deal with this, assuming you have PHP 4.2.0 or later (and I'd guess that everyone is by now), then just go into System Administration | Preferences | General Settings, and about halfway down the page, there are three settings on sessions, the first being Use Custom Sessions (turn this on), the second being the name (whatever you want), and the third is how long the session will be valid (between idle times) in minutes. This is probably what you are looking for.